For spinal support or correction, a device comprising one or more support bars or plates positioned along the spinal column is used, and fixed to certain vertebrae by implants. Said implants are fixed at one end to the plate and at the other end to the vertebrae by bone anchorage means, for example a threaded part screwed inside the actual vertebra.
In such devices, it is known to use a plate comprising several holes, to join the implants fixed to several vertebrae, as described in the patent FR2726171, for example. Said bars then surround or pass through the head of the screw and are locked with a nut screwed onto said head.
However, such a device requires that the implants be fixed and completely clamped before the plate is positioned. Therefore, in the event of delicate operative conditions, it is difficult to successfully position the plate very close to the spine. This problem arises for example when the shape of the spine comprises too many irregularities, due to spinal displacement or deformation or in the presence of outgrowths such as osteophytes. There are similar problems in the case of implantation by the anterior approach, i.e. via the front of the body or on the front face of the spine. Indeed, the anatomical conditions in this case frequently only leave space for a compact size. In addition, it is often necessary to work by means of endoscopy in this case, which renders the operation difficult and gives a less satisfactory view of the implant insertion depth.
A device according to the prior art also requires that the clamping nut only be fitted on the screw after the screws and the plate have been positioned. Therefore, said nut can only be inserted onto the screw head during the operation, with all the difficulties and risks of loss that may be caused by handling and assembling a small part inside a human body. This operation is all the more problematic when said operation is conducted by means of endoscopy, for example when it is necessary to implant via the anterior approach, i.e. via the front of the body or on the front face of the spine.
In some cases, to enable subsequent consolidation of the attachment between the implant and the vertebra, an implant composed of a so-called “rehabitable” screw is used, i.e. a hollow screw wherein the inside communicates with the outside via openings passing through the threaded wall. During the screwing into the vertebra, part of the bone substance penetrates inside the screw. Over time, the bone substance fuses between the inside and outside of the screw via these openings, thus forming a consolidation over time.
In this way, the patent FR 2726171 discloses a hollow screw wherein the openings are produced by cutting on the inner surfaces of said screw longitudinal grooves which cut into the base of the outer threading. However, during positioning or subsequently, such a screw may form anchoring which is not sufficiently strong and is liable to be dislodged or torn from the vertebra wherein it is implanted.